The use of spacers or standoffs in construction of electronic gear is well known. Typically, such standoffs are made of metal, nylon, or some other plastic material and are cylindrical in external shape and annular in cross sections perpendicular to their longitudinal axes. They provide a convenient way of providing predetermined spaces between mechanical elements and an electronic assembly. For example, it is common to use spacers to set the soldered side of a circuit board away from a metal equipment enclosure by at least a predetermined distance to avoid shorting of soldered contacts on the circuit board to or through the enclosure. Additionally, standoffs help provide required minimum spacings to allow adequate air flow through electronic devices housed in typical equipment enclosures to allow adequate heat dissipation for active components of the circuit.
The need for the present invention arose in connection with a need to assemble a piece of electronic equipment in a manner in which all of the electronics of the main body of the unit, i.e., all but those associated with a front panel connected through a plug and ribbon cable, were to be assembled in a subassembly which could be quickly dropped into and attached to an equipment enclosure. While the present invention has utility in the construction of many devices, the environment which led to the need for same was the construction of a 9600 bit per second modem implementing Recommendation V.32 of the CCITT. Such devices are relatively complex and normally include a large number of components, due primarily to the stringent digital signal processing requirements of same. The particular application was one which required two circuit boards to be assembled one over the other and spaced apart a predetermined distance.
As is well known to those skilled in the art, circuit board "real estate" is a precious commodity in the modern world of construction of physically attractive, commercially practical electronic apparatus. This is particularly true in connection with computer peripheral devices which are experiencing a drive toward miniaturization which is both technically driven (i.e., more miniaturization is possible) and market driven, in that customers are demanding greater functionality out of increasingly diminutive boxes.
Anyone who has disassembled a piece of electronic equipment of any complexity is quite familiar with the common phenomenon of lost screws, spacers, and lock washers, and the sudden collapse of the structure due to everything finally giving away at once as the device is disconnected from the enclosure and the enclosure is opened. While this rarely creates problems of reassembly for the skilled technician familiar with the product, it, of necessity, leads to products that are more difficult and time consuming to assemble at the point of initial manufacture.
For example, if a device employing a pair of circuit boards, one over the other, is to be assembled using conventional spacers where a common screw is passed through aligned holes in the circuit boards, a conventional spacer separating same, and one hole of at least one side of the equipment enclosure, the following steps are required as a practical matter. The screw must first be inserted through the panel of the equipment enclosure nearest the head of the screw. Assume for a moment that this is the bottom portion of the equipment enclosure. First, any spacer or standoff necessary to keep the bottom of the lower circuit board off the enclosure must be dropped over the shank of the screw. Next, the lower circuit board must be put in place, followed by a standoff, followed by the upper circuit board, which in turn is followed by any standoffs or other devices used to keep the upper circuit board from sliding along the screw, or contacting the upper portion of the enclosure. Typically, the upper portion of the equipment enclosure is then placed over the entire assembly and it is inverted in order to rotate the screw to get its threads to engage a threaded and mating hole connected to the upper portion of the enclosure. Those familiar with this process will quickly appreciate that if one of the screws becomes dislodged during the process of lifting and inverting, it will often fall out of the enclosure leading to a situation in which the spacers which were journaled around the screw being quickly scattered about the enclosure, and the process must be repeated. Naturally, the same conditions prevail during disassembly for purposes of repair or the like.
Personnel of applicants' assignee, including the present inventors, set out to design apparatus for assembling an electronic product using at least two circuit boards which would overcome these problems with the prior art and make the final assembly process quicker and more reliable. Additionally, it was desired to provide a set of assembly components, and a method of assembly which would keep the main module of the electronic device intact when the enclosure was removed and minimize the time required to reassemble the entire product back into the enclosure.
Additionally, in the environment of the preferred embodiment, the assembly is finally put together by passing a screw through one panel of the equipment enclosure, a standoff integrally formed with the panel, both circuit boards and the standoff of the present invention, and onto a threaded female member embedded in the other panel of the enclosure. This screw is ultimately tightened with significant force so that the two opposing sections of the equipment enclosure squeeze the balance of the assembly and place the standoff under a significant amount of stress. Part of the stress results from the use of the novel and improved grounding and tolerance correction clip of the present invention.
Additionally, it is known to those skilled in the art that there is a strong need for good design techniques which will prevent the free space radiation of radio frequency interference from modern electronic devices. This is particularly true with respect to computer peripheral devices, such as modems, which use significant amounts of high speed digital circuitry in the construction thereof. In the United States of America, there are stringent legal requirements in the form of upper limits on free space radio frequency emission allowable from computer peripheral devices over specified bandwidths which must be met before the product can be legally sold in the U.S.A.
Part of the basics of good RFI suppression design practice for computer peripherals includes the use of wide foil areas of grounded conductors around the periphery of circuit boards. It is highly desirable to provide good secure ground connections between these areas and a case ground electrical point, normally manifesting itself as electrically conductive shielding material disposed on the interior of the enclosure for the equipment. In other cases, the shielding effect of the case is accomplished by making all or the substantial majority of the case from electrically conductive material.
A portion of the present invention constitutes an improved grounding and spring clip which serves dual functions of providing a dependable ground connection between grounded conductors on the periphery of a circuit board and compensating for various accumulated mechanical tolerances in the assembly of the final device. The use of this clip, under certain tolerance conditions, can lead to significant stress on the standoff.
The net result is that the standoff of the present invention is subject to a significant stress in the form of being longitudinally compressed in the final assembly. In the environment in which the present invention is used, a fairly wide range of stresses must be tolerated by the standoff in order to accomplish the goal of a tight and dependable final assembly with a particular sized reveal being provided at the mating edges of the two sections of the equipment enclosure.
The inventors of the present invention discovered that conventionally available standoffs tended to deflect radially, or bow, under some of the longitudinal compressions reqired to make the final assembly, depending on the necessary compression to provide the proper reveal which is a function of the cumulative tolerance of the thicknesses of the circuit boards, and the height of the standoffs formed integrally with the enclosure panels.
Additionally, it was desired to provide a standoff which would serve the function of holding two or more circuit boards together in a single subassembly when the screws passing through the standoffs are removed and one of the boards is removed from the enclosure.
There is also a need in the art for a clip device which will serve multiple functions within an electronic assembly. In particular, it is desirable to have a clip device which can provide good ground connections on one or more circuit boards used in an assembly. It is additionally preferable to have the same device be able to compensate for the variations in manufacturing size of components, within specified tolerance range, including the thickness of circuit boards, the length of the shank area of the standoff of the present invention, and certain spacings within the equipment enclosure itself.
Furthermore, in prior art devices using plastic enclosures and/or standoffs, through which the mechanical assembly bolts holding the entire apparatus together pass, the phenomenon of stress relaxation has been observed. Stress relaxation is a phenomenon of plastic materials which occurs when they are put under a stress load for long periods of time. If, for example, an elongated machine screw and nut are used to tighten an electronic assembly through a plastic enclosure and plastic standoffs, it will be found, after passage of a certain period of time, that the mechanical stress intially generated by the tightening of the screw will have relaxed. This may lead to a loosening of components within the equipment which can ultimately lead to the machine screw and bolt becoming separated or a generally loose feeling about the equipment. This can lead customers to believe that same is shoddily manufactured, when in fact, the apparatus is properly made and the unavoidable mechanical phenomenon of stress relaxation is simply manifesting itself.